1. Field of the Invention
The invention relates to synthetically generated displays for aircraft flight instrumentation, and more particularly to hybrid cathode ray tube displays using digitally generated rasters and stroke vector displays.
2. Description of the Prior Art
Stroke written cathode ray tube (CRT) displays trace the shape of figures to be presented by deflection the electron beam in a manner which connects a successive sequence of strokes, which may be straight or curved. In a raster system the beam is caused to trace a repetitive pattern of parallel scan lines and the information is presented by intensity modulating the electron beam at the appropriate points along each line.
A hybrid display system includes a conventional stroke vector generator and a conventional raster symbol generator which supply sequentially a single CRT with a picture that includes both raster and stroke information. This composite display permits providing rapid update of the character symbology and a colored background with minimal requirements for memory and processing time. In hybrid CRT display systems used in applications such as aircraft instrumentation, real time high speed updates of raster and stroke symbology are required. It is also desirable to produce complex and dynamic raster symbology in addition to stroke symbology, which was not heretofore attainable with conventional raster displays. The raster symbology must be produced in an efficient manner with respect to computation time, quantity of circuitry, and power dissipation.
Digital raster display generators are known in the prior art that utilize permanently wired dedicated raster symbol generation circuitry for generating video signals during the time intervals defined by the digital circuitry generating the raster. See, for example, Display Systems Engineering, Luxenberg & Kuehn, McGraw-Hill, Inc., 1968, pp 267-269. Such systems generally utilize a unique permanently wired symbol generator for each raster symbol or pattern to be displayed. Such systems have the disadvantage of lack of flexibility, since they are not programmable, and they require large amounts of permanently wired circuitry. Consequently, this approach also requires significantly increased volume and power for the electronics. These desiderata are particularly significant in the field of airborne systems.
Other prior art digital display generators use software intensive program techiques. Software intensive techniques have a primary disadvantage of using large amounts of valuable computer time in a real-time system, where processing time is critical.
Two basic methods have been used by prior art software systems. In one approach, as outlined in High Resolution Graphics System, William Burden, Jr., Popular Computing, July 1982, pp 116-120, a full-field memory or bit-mapped technique is used, where each resolution element of the display is defined by a group of memory bits in accordance with the individual picture elements on the display screen. The picture is loaded into memory from a computer and the entire memory is read out in synchronism with the digital circuitry generating the raster. An image is produced by specifically setting, for each picture element, the color and intensity desired by writing the appropriate data into the full-field memory. The serial digital memory output words are converted to analog form and are transmitted to the display for each frame refresh. From a hardware stand-point, this approach is unattractive because of the size of the required memory and support circuitry. For example, for displays of nominal size utilizing an adequate color and contrast range, memory capacities of up to one million bits are required. Further, for dynamic symbology, the changing data for each memory element must be repetitively calculated and specifically programmed and stored in memory. This results in a prohibitively high use of processor time and a resulting image whose update rate is unacceptably slow. It may further be appreciated that because of the necessity for rapid readout of the large memory required, a high speed memory system would of necessity be utilized, which tends to be complex, expensive and critical in operation.
A second approach to raster symbol generation is provided in U.S. Pat. No. 4,070,662, Digital Raster Display Generator For Moving Displays, invented by Parm L. Narveson and assigned to the assignee of the present invention. In this approach, the face of the display device is divided into a matrix of cells. A symbol library contains a number of symbols or bit patterns which may be placed into each cell as desired. While reducing the storage requirements for pattern generation, the memory required, along with the support hardware necessary to access and control the memory, makes the cell approach a moderately expensive implementation. Further, the cell approach has not proved to be well suited for the dynamically changing symbology commonly found in flight display applications. Although such movement can be accomplished, it can be done only to a limited extent in practice and may require a significant amount of processor time to calculate the appropriate cell and symbol definitions.